Digitally modeling the deformation of gingival tissue during orthodontic treatment

ABSTRACT

A computer obtains a digital model of a patient&#39;s dentition, including a dental model representing the patient&#39;s teeth at a set of initial positions and a gingival model representing gum tissue surrounding the teeth. The computer then derives from the digital model an expected deformation of the gum tissue as the teeth move from the initial positions to another set of positions.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/280,556, filed Oct. 24, 2002, now U.S. Pat. No. 6,685,470, which wasa continuation of U.S. patent application Ser. No. 09/311,716, filed May14, 1999, now U.S. Pat. No. 6,514,074 the full disclosures of which areincorporated herein by reference.

This application is related to U.S. patent application Ser. No.09/264,547, filed on Mar. 8, 1999, and entitled “Segmenting a DigitalDentition Model,” which is a continuation-in-part of U.S. patentapplication Ser. No. 09/169,276, filed on Oct. 8, 1998, and entitled“Computer Automated Development of an Orthodontic Treatment Plan andAppliance,” now abandoned, which claims priority from PCT applicationPCT/US98/12681, filed on Jun. 19, 1998, and entitled “Method and Systemfor Incrementally Moving Teeth,” which claims priority from U.S. patentapplication Ser. No. 08/947,080, filed on Oct. 8, 1997, now U.S. Pat.No. 5,975,893 which claims priority from U.S. provisional applicationNo. 60/050,342, filed on Jun. 20, 1997, all of which are incorporated byreference into this application.

This application also is related to U.S. patent application Ser. No.09/311,941, filed on May 14, 1999, now U.S. Pat. No. 6,409,504 andentitled “Manipulating a Digital Dentition Model to Form Models ofIndividual Dentition Components”; U.S. patent application Ser. No.09/169,036, U.S. Pat. No. 6,450,807 entitled “System and Method forPositioning Teeth”; and U.S. patent application Ser. No. 09/169,034, nowU.S. Pat. No. 6,471,511 entitled “Defining Tooth-moving AppliancesComputationally,” all of which are incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates generally to the fields of dentistry andorthodontics. Two-dimensional (2D) and three-dimensional (3D) digitalimage technology has recently been tapped as a tool to assist in dentaland orthodontic treatment. Many treatment providers use some form ofdigital image technology to study the dentitions of patients. U.S.patent application Ser. No. 09/169,276, incorporated by reference above,describes the use of 2D and 3D image data in forming a digital model ofa patient's dentition, including models of individual dentitioncomponents. That application also describes using the digital dentitionmodels in developing an orthodontic treatment plan for the patient, aswell as in creating one or more orthodontic appliances to implement thetreatment plan.

BRIEF SUMMARY OF THE INVENTION

The invention provides computer-automated techniques for digitallymodeling the gingival tissue of an orthodontic patient, includingdeformation of the gingival tissue during orthodontic treatment. Thesetechniques are useful, for example, in creating orthodontic appliancesthat carry out the orthodontic treatment while fitting securely over thepatient's teeth and gums. Modeling the gums and deformation of the gumshelps ensure that the orthodontic appliances do not press too tightlyagainst the patient's gums and cause discomfort or pain. Modeling howthe gums react to treatment also provides a more complete understandingof what to expect during treatment. Moreover, computer-generated imagesand animations of the dentition during treatment are more complete andaesthetically pleasing when the gums are included.

The invention involves using a computer to develop a course of treatmentfor an orthodontic patient. In one aspect, the computer first obtains adigital model of a patient's dentition, including a dental modelrepresenting the patient's teeth at a set of initial positions and agingival model representing gum tissue surrounding the teeth. Thecomputer then derives from the digital model an expected deformation ofthe gum tissue as the teeth move from the initial positions to anotherset of positions.

In another aspect, the computer obtains a data set representing thepatient's dentition, including a digital model of the patient's teeth ata set of initial positions and a first digital gingival modelrepresenting the gingival tissue surrounding the teeth at the set ofinitial positions. The computer then derives from the data set a seconddigital gingival model for use in modeling deformation of the gingivaltissue as the teeth move. The computer, with or without humanassistance, manipulates the second gingival model to model deformationof the patient's gums.

Other embodiments and advantages are apparent from the detaileddescription and the claims below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C show the arrangement of a patient's teeth at aninitial stage, an intermediate stage, and a final stage of orthodontictreatment.

FIG. 2 shows a partial model of a patient's dentition, including a modelof gingival tissue.

FIG. 3 shows a model of the gum line around a tooth crown.

FIG. 4 is a flow chart of a technique for creating a secondary gingivalmodel.

FIG. 5 illustrates the selection of control points in a digital model ofthe gum line of FIG. 3.

FIG. 6 illustrates the creation of two curves representing gingivalmargins among control points such as those shown in FIG. 5.

FIGS. 7A, 7B, and 7C illustrate the creation of a curve representing thebase of a gingival model and the selection of control points on thatcurve.

FIG. 8 is a partial perspective view of a secondary gingival modelformed by connecting the curves of FIG. 6 and the curve of FIG. 7C.

FIG. 9 is a flow chart of a technique for creating the secondarygingival model.

FIGS. 10 and 11 are graphs showing profile curves for use in forming thesecondary gingival model.

FIG. 12 is a flow chart of a technique for creating gum surfaces in thesecondary gingival model.

FIG. 13 shows a graphical user interface component that allows a humanoperator to modify the shapes of the profile curves of FIGS. 10 and 11.

FIGS. 14, 15, and 16 are a graph and flow charts illustrating onetechnique for modeling the deformation of gingival tissue during thecourse of orthodontic treatment.

FIG. 17 is a flow chart of an alternative technique for modelingdeformation of gingival tissue.

FIGS. 18A, 18B, and 19 illustrate this alternative technique.

FIGS. 20 and 21 are flow charts describing specific embodiments of thisalternative technique.

DETAILED DESCRIPTION OF THE INVENTION

U.S. patent application Ser. Nos. 09/169,276, 09/264,547, and 09/311,941(now U.S. Pat. No. 6,409,504) describe techniques for generating 3Ddigital data sets containing models of individual components of apatient's dentition. These data sets include digital models ofindividual teeth and the gingival tissue surrounding the teeth. Theseapplications also describe computer-implemented techniques for using thedigital models in designing and simulating an orthodontic treatment planfor the patient. For example, one such technique involves receiving aninitial data set that represents the patient's teeth before treatment,specifying a desired arrangement of the patient's teeth after treatment,and calculating transformations that will move the teeth from theinitial to the final positions over desired treatment paths. U.S.application Ser. No. 09/169,276 also describes the creation of data setsrepresenting the tooth positions at various treatment stages and the useof these data sets to produce orthodontic appliances that implement thetreatment plan. One technique for producing an orthodontic applianceinvolves creating a positive mold of the patient's dentition at one ofthe treatment stages and using a conventional pressure molding techniqueto form the appliance around the positive mold. The design oforthodontic appliances from digital dentition models is described inU.S. application Ser. No. 09/169,034.

FIGS. 1A, 1B, and 1C show a patient's dentition at three stages during acourse of treatment. FIG. 1A illustrates the initial positions of thepatient's teeth before treatment begins. A digital model of the teeth atthese initial positions is captured in an initial digital data set(IDDS). The digital model contained in the IDDS also includes portionsrepresenting gingival tissue surrounding the patient's teeth. A computersegments the IDDS into digital models of individual teeth and thegingival tissue.

FIG. 1B illustrates the patient's teeth at an intermediate stage in thetreatment process, and FIG. 1C illustrates the teeth at their finalpositions. In many cases, a human operator manipulates the digitalmodels of the patient's teeth to prescribe the final tooth positions.The computer then calculates one or more of the intermediate positions,taking into account any constraints imposed on the movement of the teethby the human operator or by the natural characteristics of the teeththemselves. The computer also accounts for any collisions that mightoccur between teeth as the teeth move from one treatment stage to thenext. Selecting the final and intermediate tooth positions and thetreatment paths along which the teeth move is described in more detailin one or more of the applications incorporated by reference above.

FIG. 2 illustrates a portion of a typical digital dentition model 110derived from the IDDS. The dentition model 110 includes models ofindividual teeth 120 and a model of the patient's gums 140. Varioustechniques for creating models of gum tissue and individual teeth fromthe IDDS are described in U.S. application Ser. Nos. 09/264,547 and09/311,941.

FIG. 2 also shows a portion of a another gingival model 200 (a“secondary” gingival model), which is constructed to overlie thegingival model 140 derived from the IDDS (the “primary” gingival model).The computer uses the secondary gingival model 200, as described below,to model the deformation of the gingival tissue around the patient'steeth as the teeth move from the initial to the final positions. Thisensures that orthodontic appliances made from positive molds of thepatient's dentition fit comfortably around the patient's gums at alltreatment stages. The secondary gingival model 200 also adds thicknessto the gum model, which ensures that the orthodontic appliances do notpress too tightly against the patient's gums.

FIG. 3 shows a tooth crown 125 and a corresponding gingival margin 160around the tooth crown 125. The primary gum model 140 derived from theIDDS includes information identifying the gingival margin 160surrounding each tooth crown 125 in the dentition model 110. Thecomputer uses this information, as described below, to create thesecondary gingival model 200 automatically.

FIGS. 4-16 illustrate one technique for creating the secondary gingivalmodel 200. In short, a computer, under control of a program implementingone or more techniques of the invention, creates curves representing thepatient's gum lines and the base of the secondary model and then createssurfaces joining these curves. These surfaces become the gum surfaces inthe secondary gingival model 200.

Referring to FIGS. 4 and 5, the computer first identifies each gingivalmargin 160 in the dentition model (step 500) and selects points 205A-Flying on the gingival margin 160 (step 502). The computer selects atleast one point lying on the buccal surface 210 of the gingival margin160 and at least one point lying on the lingual surface 215. In theexample shown here, the computer selects six points 205A-F, three oneach of the buccal and lingual surfaces. One technique for selectingthese six points involves projecting the gingival margin 160 onto an x-yplane, selecting the point 205A at which the gingival margin 160intersects one axis 220 of the plane, and then selecting the remainingpoints 205B-F at intervals of 60□ around the gingival margin 160. Insome embodiments, the computer varies the interval between the selectedpoints or allows a human operator to select the positions of the points.In many cases the computer selects more than three points, often manymore than three. The selected points can serve as control points thatallow the human operator to manipulate the shape of the secondarygingival model 200, as described below.

Referring also to FIG. 6, once the computer has selected the points oneach gingival margin 160, the computer uses the selected points tocreate two curves 230, 235 representing the gum lines on the buccal andlingual sides of the dentition model (step 504). These curves 230, 235extend along the buccal and lingual surfaces of the teeth in thedentition model, beginning at the rear molar on one side of thepatient's mouth and ending at the rear molar on the other side of thepatient's mouth. The computer forms the curves by fitting one splinecurve among the selected points on the buccal sides of the gingivalmargins and by fitting another spline curve among the selected points onthe lingual sides of the gingival margins. The spline curves may or maynot pass though all of the selected points from one model to the next.

Referring also to FIGS. 7A, 7B, and 7C, the computer then creates acurve 240 that defines the base of the secondary gingival model 200(step 506). One technique for creating the base curve 240 involvescreating a plane 245 that intersects the primary gingival model 140 atsome predetermined distance x below the teeth 120. This plane 245 isusually normal to the z-axis 250 of the dentition model 110 and roughlyparallel to the bottom surface 255 of the dentition model 110. The basecurve 240 occurs where the plane 245 intersects the surface of thedentition model 110. The distance x between the plane 245 and the teeth120 varies from model to model, but a distance on the order of onemillimeter is common. FIG. 7B shows that, in some embodiments, thesecondary gingival model 200 often is large enough to cover only aportion of the primary gingival model 140. In some of these embodiments,the secondary model 200 covers only the portion of the gums that comeinto contact with the orthodontic appliances.

The computer optionally defines control points 260 on the base curve 240of the secondary gingival model 200 (step 508). These control points 260allow a human operator to modify the shape of the secondary gingivalmodel 200. In the example shown, the computer defines eight controlpoints on each of the buccal and lingual sides of the base curve 240.

Referring also to FIG. 8, the computer completes the secondary gingivalmodel by creating surfaces 265, 270, 275 that connect the gingivalcurves 230, 235 to each other and to the base curve 240 (step 510). FIG.9 shows one technique for creating these surfaces. The computer firstselects points 280A-F on each of the curves 230, 235, 240 (step 520).The computer then creates curves 285A-D between these points (step 522)and creates surface segments 290A, 290B that connect these curves (step524). The points 280A-F selected by the computer may or may not be thecontrol points 205A-F, 260 discussed above. Each point on one of thegingival curves 230, 235 has two corresponding points, one lying on theother gingival curve and one lying on the base curve 240. Each point onthe base curve 240 has one corresponding point, which lies on thenearest one of the gingival curves 230, 235.

FIGS. 10 and 11 show profile curves 295A, 295B, 295C that are used insome cases to create the curves 285A-D of in FIG. 8. The profile curvesrepresent the shapes of the gingival surfaces 265, 270, 275 between twocorresponding points. As shown in FIG. 12, the computer inserts theprofile curve 295A of FIG. 10 between pairs of points lying on thegingival curves 230, 235 to form the top surface 265 of the secondarygingival model 200 (step 530). The computer inserts the profile curves295B, 295C of FIG. 11 between points lying on the gingival curves 230,235 and points lying on the base curve 240 to form the buccal andlingual surfaces 270, 275 of the secondary gingival model 200 (step536). The computer chooses one of the curves 295B, 295C of FIG. 11 basedupon the type of tooth that resides in the area in which the curve is tobe inserted (step 532). For example, teeth at the front of thedentition, such as incisors, typically produce less curvature in the gumsurface than teeth at the rear of the dentition, such as molars andcanine teeth. Therefore, the computer selects the profile curve 295Bwith the flatter profile to model the gum surfaces around front teethand selects the profile curve 295C with the more rounded profile tomodel the gum surfaces around rear teeth.

In some embodiments, each of the profile curves 295B, 295C of FIG. 11 isassociated with one type of tooth, such as a front incisor or a rearmolar. In these situations, the computer creates curves for gum surfacesadjacent to other types of teeth by interpolating between the twoprofile curves 295A, 295B (step 534). For example, in some cases thecomputer averages the two profile curves 295B, 295C of FIG. 11 to createa custom curve 295D representing the gum surfaces in the vicinity of abicuspid. Other alternatives include storing a unique profile curve foreach tooth or each type of tooth in the dentition model.

The computer also allows a human operator to adjust the shapes of theprofile curves, and thus the gum surfaces, in some embodiments (step538). FIG. 13 shows a graphical user interface component, in the form ofa slide bar 300, that allows the operator to adjust the curvature ofeach of the profile curves 295A, 295B, 295C. Adjusting the position ofthe slide element 305 in the slide bar 300 changes the tangent vectorsalong the profile curves, which in turn changes the shapes of thecurves. Other alternatives include placing control points along thecurves and allowing the human operator to adjust the positions of thecontrol points.

FIGS. 14 and 15 show how the secondary gingival model 200 is used tomodel deformation of the gum tissue during the treatment process. Asdescribed in U.S. application Ser. No. 09/169,276, incorporated byreference above, the computer, either with or without human assistance,creates models of the patient's teeth at key treatment stages, known askey frames, between the initial and final positions. The computer alsoderives treatment paths along which the patient's teeth will move ingoing from the initial to the final positions. The treatment pathsinclude both translational and rotational components that create theforces necessary to move the teeth to the desired positions. Thetreatment path for each tooth fits among the desired tooth positions ateach of key frames but need not actually pass through all of the desiredpositions at the key frames. Models of the patient's dentition atvarious intermediate stages along the treatment path are used to createthe orthodontic appliances that carry out the treatment plan.

In modeling gum deformation, the computer selects one or more points 310along the gingival curves 230, 235 and the base curve 240 of thesecondary gingival model (step 540) and calculates the positions of eachpoint 310 at the initial and final stages SO, SF and at each of the keyintermediate stages SIx, or key frames (step 542). In most cases, thepoints selected by the computer are the control points described above.The computer then calculates a path 315 along which the point 310 willtravel. In general, the path 315 follows the curvature of the tooth thatis closest to the point 310 in the dentition model. The computerdetermines the position of the point 310 at any given intermediate stageby the interpolating linearly along the path 315. For example, for atreatment plan in which the initial stage is Stage 1 and the first keyframe occurs at Stage 10, the position of the point 310 at intermediateStage 5 is the midpoint of the curved path 315 between Stage 1 and Stage10. The computer generates gingival models at each of these stages byinserting profile curves and surfaces as described above.

FIG. 16 illustrates one technique for calculating the positions of theselected points on the gum lines at the key frames. This techniqueassumes that the selected points travel with the teeth from the initialpositions to final positions. The computer first identifies the toothclosest to each point (step 550). The computer then monitors thetransformations applied to each tooth in moving the tooth among the keystages to its final position (step 552). The computer applies thesetransformations to the corresponding points on the gum lines (step 554).

FIGS. 17, 18A, and 8B illustrate another technique for modeling thedeformation of gum tissue. This technique involves dividing thesecondary gingival model 200 into multiple pieces 320, 325, 330 bycutting very thin slices 335, 340 in the secondary gingival model 200between pairs of adjacent teeth 345/350, 345/355 (step 560). Dividingthe gingival model 200 in this manner allows one or more of the pieces320, 325, 330 to move with a corresponding tooth 345, 350, 355. Thistechnique is used most commonly to model the deformation of gum tissuearound teeth that move relatively little between two treatment stages.

For each piece 320 of the secondary gum model that is to move with atooth 345, the computer “attaches” the piece 320 to the digital model ofthe tooth 345 (step 562). The computer monitors the transformationsapplied to the tooth 345 (step 564) and applies each of thesetransformations to the attached piece 320 of the gingival model (step566). As the piece 320 of the gingival model 200 moves with the toothmodel, the slices 335, 340 between the pieces 320, 325, 330 of the gummodel 200 grow in size, creating gaps between the pieces. When the piece320 finally is detached from the corresponding tooth model (step 568),the computer “stitches” the secondary gingival model 200 by creatingsurfaces 360, 365 to fill the gaps (step 570).

FIGS. 19 and 20 show one technique for creating a stitching surface 400in the secondary gingival model 200. For each slice 335 to be stitched,the computer selects points 380 on the edges 370, 375 of the two pieces320, 325 that bound the slice 335 (step 580). The computer then createsline segments 385 to form triangles bounded by the selected points 380(step 582). Each of the triangles is bounded by two points on one of theedges 370, 375 and one point on the other edge. The computer thendivides each triangle into four smaller triangles by selecting points390 at or near the centers of the line segments 390 and creating smallerline segments 395 that extend between pairs of the newly selected points390 (step 384). The result is a triangular surface mesh in which eachpoint that does not lie on one of the edges 370, 375 (each “centervertex”) is shared by six of the smaller triangles. For example, thecenter vertex labeled 405B is shared by the six smaller triangleslabeled T1, T2, T3, T4, T5, T6. Each vertex that lies on one of theedges 370, 375 is shared by three of the smaller triangles.

In some embodiments, the computer adjusts the shape of the stitchingsurface 400 by moving the center vertices 405A, 405B, 405C in adirection that is roughly normal to the stitching surface 400 (step586). FIG. 21 illustrates one technique for adjusting the curvature ofthe stitching surface 400. The computer first calculates the normals forthe six triangles that share each center vertex 405A, 405B, 405C (step590). The computer then averages these normals to find an “averagenormal” at the center vertex (step 592). The computer then moves thecenter vertex along the average normal by a selected amount (step 594).In some cases, the computer receives instructions from a human operatorto adjust the shape of the stitching surface in a certain manner (step596). For example, one embodiment displays the center vertices ascontrol points, which the user manipulates with an input device such asa mouse.

Some implementations of the invention are realized in digital electroniccircuitry, such as an application specific integrated circuit (ASIC);others are realized in computer hardware, firmware, and software, or incombinations of digital circuitry and computer components. The inventionis usually embodied, at least in part, as a computer program tangiblystored in a machine readable storage device for execution by a computerprocessor. In these situations, methods embodying the invention areperformed when the processor executes instructions organized intoprogram modules, operating on input data and generating output. Suitableprocessors include general and special purpose microprocessors, whichgenerally receive instructions and data from read-only memory and/orrandom access memory devices. Storage devices that are suitable fortangibly embodying computer program instructions include all forms ofnon-volatile memory, including semiconductor memory devices, such asEPROM, EEPROM, and flash memory devices; magnetic disks such as internalhard disks and removable disks; magneto-optical disks; and CD-ROM.

The invention has been described in terms of particular embodiments.Other embodiments are within the scope of the following claims.

1. A computer-implemented method for use in developing a course oftreatment for an orthodontic patient, the method comprising: scanning apatient's teeth or a physical model thereof to obtain data; receiving ina computer the data obtained by the scanning; obtaining in the computera digital model of a patient's dentition, including a dental modelrepresenting the patient's teeth at a set of initial positions and agingival model representing gum tissue surrounding the teeth; andderiving in the computer from the digital model data representing anexpected deformation of the gum tissue as the teeth would move from theinitial positions to another set of positions.
 2. The method of claim 1,wherein the computer derives the expected deformation of the gum tissueby: separating from the gingival model a portion that represents gumtissue surrounding a particular tooth; and subjecting the separatedportion to at least one force that is applied to the particular tooth.3. The method of claim 2, wherein the computer reconnects the separatedportion to an adjacent portion of the gingival model after subjectingthe separated portion to the transformation.
 4. The method of claim 3,wherein, in subjecting the separated portion to at least one force, thecomputer creates a gap between the separated portion and the adjacentportion of the gingival model, and, in reconnecting the separatedportion to the adjacent portion, the computer creates a stitchingsurface to fill the gap.
 5. The method of claim 4, wherein the computeradjusts the shape of the stitching surface to alter the shape of the gumtissue in the reconnected gingival model.
 6. The method of claim 5,wherein, in adjusting the shape of the stitching surface, the computerreceives instructions from a human operator concerning the shape of thestitching surface.
 7. The method of claim 4, wherein the stitchingsurface is bounded by two curves representing edges of the separatedportion and the adjacent portion of the gingival model.
 8. The method ofclaim 7, wherein, in creating the stitching surface, the computerselects points on the curves and connects the points to form trianglesrepresenting a surface mesh.
 9. The method of claim 8, wherein thecomputer adjusts the shape of the surface mesh by moving a vertex sharedby multiple ones of the triangles along a line.
 10. The method of claim9, wherein, in moving the vertex, the computer calculates a normal linefor each of the triangles that share the vertex and calculates anaverage of the normal lines.
 11. The method of claim 8, wherein thecomputer divides each of the triangles into smaller triangles to form afine surface mesh.
 12. The method of claim 11, wherein the computerdivides the triangles such that each of the smaller triangles has atleast one vertex shared by five more of the smaller triangles.
 13. Themethod of claim 1, wherein, in deriving an expected deformation of thegum tissue, the computer selects a point in the gingival model andderives motion of the point as the teeth move from the initial set ofpositions to the other set of positions.
 14. The method of claim 13,wherein, in deriving motion of the point, the computer attaches thepoint to a model of a corresponding tooth in the dental model andsubjects the point to transformations applied to the correspondingtooth.
 15. The method of claim 13, wherein the point lies on a gingivalmargin, at which the gum tissue meets one of the teeth.
 16. The methodof claim 13, wherein the computer creates another gingival modelrepresenting the gum tissue surrounding the teeth at the other set ofpositions.
 17. The method of claim 16, wherein, in creating anothergingival model, the computer selects points in the gingival model,derives positions for the points when the teeth are at the other set ofpositions, and creates a curve that connects the points at the derivedpositions.
 18. The method of claim 17, wherein, in creating the curve,the computer selects the curve from a group of curves that havepredetermined profiles.
 19. The method of claim 18, wherein, inselecting the curve, the computer determines which type of tooth isnearest the points in the dentition model and selects a curve associatedwith the type of tooth that is nearest the points.
 20. The method ofclaim 17, wherein, in creating the curve, the computer interpolatesbetween two curves having predetermined shapes.
 21. The method of claim17, wherein the computer receives an instruction from a human operatorto modify the shape of the curve.